This invention relates generally to data processing systems and more particularly to improved means and methods for overcoming the deleterious effects of propagation time variations in a synchronous digital data processing system.
A particular problem which is caused by propagation time variations in a synchronous data processing system occurs in connection with the design of a clock distribution system. For example, propagation time variations can produce significant skewing of the clocks applied to different parts of the system. In order to prevent this skewing from creating timing problems, a conventional solution is to add the maximum skew to the minimum system cycle time so as to thereby guarantee that all data signals arrive at their destination storage elements (e.g., flip-flops) before the clock does. In today's high performance systems (either computers or other systems designed using digital devices), this increase in cycle time can have a significant deleterious effect on system speed.
A primary cause of skew in a data processing system is a result of propagation time variations which occur between integrated circuit chips due to manufacturing process tolerances. This is a particularly sensitive problem in the case of clock distribution circuitry, since inter-chip propagation time delays will produce skews in the clocks distributed throughout the system.
One solution to the skew problem is to improve chip fabrication processes to make chips more uniform so that tolerances from chip-to-chip are smaller. However, the increased cost that would be involved makes this solution economically impractical.
Another type of solution which has been employed to minimize skew is to provide for manual (or operator-controlled) adjustment of the clock distribution system, such as disclosed for example in U.S. Pat. No. 4,447,870 for "APPARATUS FOR SETTING THE BASIC CLOCK TIMING IN A DATA PROCESSING SYSTEM", issued May 8, 1944, S. A. Tague, et al., inventors. Besides the inconvenience of having to provide manual or operator-controlled adjustment, this solution is also expensive because of the increased labor and/or set-up which would be required.
It is to be noted that clock skew can also present problems in connection with communication receiver circuitry where signals may be received at times which are not properly in phase with a system clock. Special synchronizing techniques have been developed for handling asynchronous received signals, such as disclosed, for example, in U.S. Pat. No. 3,908,084 for "HIGH FREQUENCY CHARACTER RECEIVER" issued Sept. 23, 1975, P. R. Wiley, inventor. However, because of the different factors involved, such techniques are not appropriate for solving the inter-chip propagation time variation problem to which the present invention is directed.